RNA tumor virus gene expression will be studied in both uninfected and infected avian cells. Approaches used will include (1) the analysis of endogenous proviral DNAs, identified by restriction mapping, and elucidation of the transcriptional patterns of these proviruses; (2) sequence analysis of critical regions of viral mRNAs that might be involved in processing; (3) attempts to demonstrate processing in vitro, and to isolate the enzyme(s) responsible for this activity; and (4) analysis of low molecular weight RNAs, which are present in infected and uninfected cells, and may be transcribed from a promoter located near the 3' end of the viral genome.